L. N. Dyakonova-Schultz

On the Method for Obtaining Cinnamic Acid

From the Laboratory of Organic Chemistry of the Siberian Chemical-Technological Institute.

The present work was undertaken with the aim of reducing the duration of heating in the Perkin reaction for obtaining cinnamic acid and of improving the yield of the product. In the literature¹⁾ there are several methods for obtaining cinnamic acid, but the generally accepted one is Perkin’s method. According to Perkin’s method, cinnamic acid is obtained by heating 2 parts of benzaldehyde, 1 part of sodium acetate, and 3 parts of acetic anhydride in an oil bath at 180° for 8 hours. By replacing the cation of sodium acetate with the more energetic potassium cation, and by changing the molecular ratios of the components—namely, taking benzaldehyde 1 mole, acetic anhydride 1.6 moles, and potassium acetate 1.5 moles—and heating at 168°, it proved possible to reduce the duration of heating from 8 hours to 2 hours with a cinnamic-acid yield of up to 78.5%, and, when heating for one hour, with a yield of up to 64.4%. Under the indicated conditions the cinnamic acid is obtained pure and does not require purification. In addition to the experiments with potassium acetate, experiments were also carried out with potash.²⁾

From the experiments conducted with potash, it was found that cinnamic acid is obtained in a yield of 71.4% when heated for 1 hour at 163° and

¹⁾ W. Perkin, j. ch. Soc. 1887. I. 390; j. Th. 1887, 789
M. Conrad und C. Bischoff, Ann. Chem. Pharm. 204, 187.
R. Fittig, Ann. Chem. Pharm. 195, 171; 216, 115; 227, 48; B. 14, 1824; 16, 1436
F. Tiemann und R. Kraatz, B. 15, 2061; 10, 68.
A. Michael, B. 34, 918; jour. pr. ch. (2) 60
J. Nef, Ann. 298, 309, 277
A. Geuther, Ann. 106, 251
H. Meyer und R. Beer, C. 1913. I 2121
J. Reich und P. Chaskelis, C. 1916 II 905. Bull Soc. chem. de France. 19, 287
Sishibhushann Dutt, C. 1925-II 1852
B. G. Bacharach, journ. Ann. Chem. Soc. 1928. 50, 3333
P. Kalnin, C. 1929 II 562; Helv ch. Acta 11, 977
Alfons M. Burger, C. 1929 I 2233
W. Rodinow, C. 1929 I 2413; jour. Am. chem. Soc. 51,847
G. Bacharach und Francis Brogan, C. 1929 I 646; jour. Am. chem. S 50, 3333
A. Koss und Pellegrini, C. 1930 II 556
S. Boxer und R. Linstead, C. 1931 II 37; jour chem. Soc. 1931, 740
W. N. Rutowski und D. A. Brodski, C. 1932 II 1074. (Russ. P. 23,396)
G. Bacharach, C. 1932 II 123 (A. P. 185, 3630)

²⁾ P. Kalnin, C. 1929 II 562; Helw. ch. Acta 11, 977

under the following molecular ratios: benzaldehyde 1 mole, acetic anhydride 2.6 moles, and potassium acetate 1.2 moles.

It should be noted that when potassium acetate is used, cinnamic acid is obtained with a slightly yellow color and requires purification.

Experimental Part.

Benzaldehyde, acetic anhydride, and potassium acetate.

A mixture of benzaldehyde, acetic anhydride (both freshly distilled), and potassium acetate (anhydrous) was heated on an oil bath in a flask with a reflux condenser equipped with a calcium chloride tube. The flask was connected to the condenser by means of a horn-shaped adapter. A thermometer was inserted into the other arm of the adapter (the mercury bulb of the thermometer was in the liquid). When the components were mixed, heating was observed. On heating to 82° the mixture begins to boil and the liquid turns yellow. At 100° foam appears. At 140° the reaction proceeds vigorously. At 152°, 40 minutes after the start of heating, a yellow crust forms on the walls of the flask. On further heating the crust increases and covers the entire surface of the liquid. After completion of the experiment and cooling, the reaction product turns into a solid mass of yellow color. The resulting product was dissolved in hot water and subjected to distillation with steam in order to remove unreacted benzaldehyde. At the bottom of the distillation flask a dark-colored oil was observed. To separate the oil, the hot solution of cinnamic acid was filtered. Concentrated hydrochloric acid was added to the filtrate until Congo paper turned blue—cinnamic acid immediately precipitated in the form of shiny plates with m.p. 133°. The experiments performed may be divided into three series. The aim of the first series of experiments was to determine the influence of the molecular ratios of the components on the yield of cinnamic acid.

The results are given in the table.

Table 1.

It is evident from the table that the molecular ratios, at the same duration of heating and temperature conditions, affect the yield of cinnamic acid. The best result was obtained with the following ratios: benzaldehyde 1 mol, acetic anhydride 1.6 mol, potassium acetate 1.5 mol.

The second series of experiments had the purpose of determining the influence of the duration of heating on the yield of cinnamic acid.

Table 2.

From the results presented it is evident that, when heating for one hour, the yield of cinnamic acid is 8.5 g (60.°/%); then, with an increase in the duration of heating, the yield slowly increases,

Fig. 1. Diagram of the results of the experiments presented in Table 2.

then slowly decreases and finally becomes stable. Thus, when heating for two and three hours, the yield of cinnamic acid is 11 g (78.5%); when heating for 5 and 6 hours, the yield of cinnamic acid is 10 g (71.4%).

The third series of experiments was intended to elucidate the influence of temperature conditions on the yield of the product.

Table 3.

As is evident from the table, the reaction temperature also affects the yield of cinnamic acid; thus, with identical molecular ratios and the same duration of heating, but under different temperature conditions, the best result is obtained at 168°—the temperature of the mixture—and an oil-bath temperature of 180–185°. With a lowering of the temperature of the mixture and of the oil bath, the yield of the product decreases.

Benzaldehyde, acetic anhydride, and potash.

Benzaldehyde, acetic anhydride (both freshly distilled), and potash (fused) were introduced into a round-bottom flask. The flask was connected with

with a reflux condenser by means of a horn-shaped forstoss. A thermometer was inserted into the straight tube of the forstoss (the mercury bulb of the thermometer was in the mixture). The mixture was heated on an air bath. On mixing the components, a slight evolution of heat was observed. At 50° an abundant evolution of CO₂ was noticed. The reaction proceeded vigorously. After 15 minutes, at 120°, the evolution of CO₂ decreased. The reaction proceeded calmly. The liquid turned yellow. At 150° a yellow crust begins to appear on the walls of the flask. The yellow color of the mixture changes to brown. On further heating the crust increases and covers the entire surface of the liquid. At the end of the experiment and after cooling, the reaction product solidified into a yellow mass. The product obtained was dissolved in hot water and subjected to steam distillation to remove unreacted aldehyde. The hot solution was filtered from oily impurities and precipitated with conc. hydrochloric acid until Congo paper turned blue. On addition of hydrochloric acid, rapid separation of cinnamic acid was observed. The product was filtered off on a Büchner funnel, washed with water, and treated with animal charcoal. After removal of the animal charcoal from the solution and cooling, cinnamic acid separates out in the form of shiny colorless plates, m.p. 133°.

The results of the experiments carried out are given in the table.

Table 1.

The best result was obtained under identical temperature conditions and duration of heating with the following molecular ratios: benzaldehyde 1 mol, acetic anhydride 2.6 mol, potash 1.2 mol.

Table 2.

From the data of the table and the diagram it is evident that at first the yield increases rapidly, then the increase proceeds slowly, and finally the yield becomes stable.

Table 3.

Raising the temperature of the mixture and of the air bath leads to a decrease in the yield of the product.

The maximum yield of cinnamic acid is obtained at a mixture temperature of 163° and an air-bath temperature of 180°.

CONCLUSIONS.

1. Of the salts used, the acetate salt gave by far the best yield.

2. Exact observance of the temperature conditions is of extremely great importance; within the range of 5–8° the yield decreases.

3. With certain molecular ratios it is not possible to attain a definite temperature, which has an adverse effect on the yield of the product.

4. Prolonging the heating is pointless. The maximum yield is obtained at two hours; conducting the experiment for a longer time sometimes even led to worse results.